Ch01 Lecture part1 video
Summary
TLDRThis astronomy lecture introduces the concept of celestial cycles, explaining the ancient Greek model of a celestial sphere encompassing fixed stars. It discusses the importance of models in scientific predictions and the distinction between daily and annual motions of celestial bodies. Constellations are described as historical signposts in the sky, with proper motion and the Earth's axial tilt affecting star positions over time. The lecture emphasizes the difference between astronomy and astrology, highlighting the scientific method and the Earth's movements as the basis for understanding the sky.
Takeaways
- 🌌 The ancient Greeks envisioned the sky as a celestial sphere with stars fixed at equal distances from Earth, akin to a glass sphere surrounding the planet.
- 📐 A model in science is a conceptual tool used to understand and predict phenomena; it's a description that can be tested and refined based on observations and predictions.
- 🔮 Constellations are patterns of stars named after mythological figures or animals, serving as signposts in the sky but not necessarily indicative of the stars' actual proximity to each other.
- 🌟 Stars exhibit proper motion, moving slightly over long periods due to their orbits around the Milky Way galaxy, though this motion is barely noticeable within a human lifetime.
- 🌍 Earth's daily motion is caused by its rotation on its axis, leading to the apparent rising and setting of celestial bodies like the Sun and Moon.
- 🌞 The Earth's axial tilt, at 23.5 degrees, affects the path of the Sun in the sky, causing seasonal variations in the Sun's position at noon.
- 🌐 The celestial equator and the ecliptic are two significant imaginary lines in astronomy; the former is the projection of Earth's equator into space, and the latter is the path the Sun appears to follow in the sky.
- 📅 The zodiac constellations, historically, function as a calendar, indicating the time of year based on the Sun's position in relation to these constellations during the day.
- 🚫 Astrology, which may use zodiac signs, is distinct from astronomy and lacks scientific validity; the script emphasizes the importance of distinguishing between the two.
- 🌠 The annual motion of Earth around the Sun results in different visible constellations at night, depending on the Earth's position in its orbit, effectively using the sky as a timepiece.
- 🌙 The Moon, like the Sun, rises and sets due to Earth's rotation, showcasing the daily motion that affects all celestial bodies as observed from our planet.
Q & A
What is the celestial sphere as described by the ancient Greeks?
-The celestial sphere is an ancient Greek concept where they imagined all the stars in the sky as being attached to a large, spherical surface that surrounds the Earth, with the stars all appearing to be at the same distance away.
What is a scientific model and why is it important?
-A scientific model is a theoretical construct that helps us understand and predict phenomena in the universe. It is important because it allows us to make predictions about future events, and if those predictions are correct, it builds confidence in the model.
What are constellations and how have they historically been used?
-Constellations are patterns of stars in the sky that have been named after mythological, historical, or imaginary figures. Historically, they have been used as signposts or markers in the sky to help navigate and locate objects in the night sky.
Why do stars in a constellation appear to be close together when they might not be?
-Stars in a constellation appear close together from our perspective on Earth, but they can actually be very far apart in space. This is because their positions in the sky are a result of our viewpoint and not their actual distances from each other.
What is proper motion and how does it relate to the stars' movement?
-Proper motion is the apparent change in a star's position in the sky over time due to its actual movement through space. All stars orbit the Milky Way, and because of their unique orbits, they slowly move relative to our viewpoint on Earth.
How does the Earth's rotation affect what we see in the sky?
-The Earth's rotation on its axis causes the daily motion we observe, such as the sunrise and sunset. As the Earth spins, different parts of the sky come into view, creating the illusion that the stars, Sun, and Moon are moving across the sky.
What is the difference between daily motion and annual motion?
-Daily motion is caused by the Earth's rotation on its axis, leading to the apparent movement of celestial bodies across the sky within a 24-hour period. Annual motion, on the other hand, is due to the Earth's orbit around the Sun, which results in different constellations being visible at different times of the year.
Why do ancient astronomers believe the Earth was stationary?
-Ancient astronomers believed the Earth was stationary because they observed the stars and other celestial bodies appearing to move around the Earth. They thought the Earth was at the center of the universe, with everything else moving around it, which was a concept known as the geocentric model.
What is the significance of the Earth's axial tilt?
-The Earth's axial tilt of 23.5 degrees relative to its orbit around the Sun causes seasonal changes and affects the Sun's position in the sky at noon. It also influences the location of the celestial equator and the ecliptic, which are important for understanding the changing sky throughout the year.
How can constellations be used as a calendar?
-Constellations can be used as a calendar because their positions in the sky change predictably throughout the year due to the Earth's orbit around the Sun. By observing which constellations are visible at certain times, one can determine the approximate time of year.
What is the difference between the zodiac constellations and astrology?
-The zodiac constellations are a set of constellations that the Sun appears to pass through in the course of a year, and they have historically been used as a calendar. Astrology, on the other hand, is a belief system that suggests these constellations influence individual personality traits and life events, which is not supported by scientific evidence.
Outlines
🌌 Introduction to Celestial Spheres and Ancient Greek Astronomy
The lecture begins with an introduction to the concept of cycles in the sky, focusing on the ancient Greek idea of a celestial sphere encompassing all stars at equal distances from Earth. The celestial sphere is likened to a glass sphere with stars affixed to it, spinning around the Earth. This model serves as a foundational framework for understanding the universe and is a key example of how scientific models work, providing predictions and insights into the nature of the cosmos. The importance of predictions in science is emphasized, as they are a means to test and validate models.
🌟 Understanding Constellations and Their Role in Astronomy
The script moves on to discuss constellations, which are described as signposts in the sky, historically used to map the heavens. Constellations like Leo and Orion are named after mythological figures, and while they appear close together to the naked eye, they can be vastly distant from one another. The paragraph also touches on the proper motion of stars, explaining that stars' positions change over centuries due to their orbits around the Milky Way, although this motion is barely perceptible within a human lifetime.
🌍 Earth's Daily and Annual Motions and Their Impact on the Night Sky
This section delves into the concepts of daily and annual motion as observed from Earth. The daily motion is attributed to Earth's rotation on its axis, causing the Sun, Moon, and stars to rise and set. Annual motion, on the other hand, is due to Earth's orbit around the Sun, leading to the visibility of different stars and constellations throughout the year. The ancient Greeks' belief in a stationary Earth with celestial bodies moving around it is contrasted with the modern understanding of Earth's active role in these motions.
📍 The Earth's Axis, Seasons, and the Celestial Equator
The Earth's tilted axis and its implications for the position of the Sun in the sky are explored. The tilt results in the Sun never being directly overhead outside the tropics and is responsible for the changing seasons. The concept of the celestial equator, an extension of Earth's equator into space, is introduced, and its tilt relative to the ecliptic—the path of the Sun—is explained. The ecliptic is identified as the apparent annual path of the Sun across the sky.
🗓 Zodiac Constellations as a Calendar in Ancient Astronomy
The lecture discusses the historical use of zodiac constellations as a calendar, with the Sun's position among these constellations indicating the time of year. The zodiac is distinguished from astrology, with the instructor emphasizing the scientific nature of astronomy. The paragraph explains how different constellations are visible at different times of the year due to Earth's orbit, and how this can be used to track the passage of time, much like a celestial calendar.
🌙 Conclusion and Preview of Upcoming Lecture Topics
The script concludes with a summary of the lecture's first part, highlighting the importance of understanding the motions observed in the sky. The instructor expresses hope for the students' enjoyment and previews upcoming topics that will further explore the movements and significance of celestial bodies.
Mindmap
Keywords
💡Celestial Sphere
💡Model
💡Constellations
💡Proper Motion
💡Daily Motion
💡Annual Motion
💡Ecliptic
💡Zodiac
💡Tilt of the Earth
💡Orbit
💡Astrology vs. Astronomy
Highlights
Introduction to the concept of cycles in the sky and the movement of celestial bodies.
Explanation of the ancient Greek idea of the celestial sphere encompassing all stars at equal distances from Earth.
Discussion on the uniform appearance of stars and the implications for their perceived distances.
Introduction to the scientific method and the role of models in understanding the universe.
Emphasis on the importance of predictions in the scientific process and the validation of models.
Brief mention of constellations as historical signposts in the sky.
Clarification that stars within constellations are not necessarily close to each other in space.
Description of proper motion of stars and their movement over extended periods of time.
The Milky Way galaxy's role in the motion of stars and their unique orbits.
Daily motion of celestial bodies due to Earth's rotation on its axis.
Annual motion resulting from Earth's orbit around the Sun and its effect on visible constellations.
Historical belief of Earth's stationary position and celestial spheres spinning around it.
Earth's axial tilt and its impact on the Sun's position in the sky at noon.
Differentiation between the celestial equator and the ecliptic in relation to Earth's tilt and orbit.
Use of constellations as a calendar to track time and seasons.
The zodiac constellations as a historical calendar, not a determinant of personal traits.
Conclusion of the lecture part with a teaser for further discussions on celestial movements.
Transcripts
okay hello everyone this is gonna be
part 1 of lecture 1 our very first
lecture in astronomy this lecture is
gonna be about cycles in the sky which
is to say how things move in the sky and
why they move the way they do so let's
first of all talk about an idea that's
been going on since the ancient Greeks
the ancient Greeks thought of the sky
around us as a celestial sphere now what
that means is basically if you imagine
all the stars so these stars you imagine
all the stars in the sky as a or in a
sphere now this idea if you go outside
tonight and look at the night sky squat
and look and well you're gonna see when
you go out and look at the night sky at
the Stars is you're gonna see all the
stars in the sky looking like stars now
what does that mean well looking like
stars well
they don't appear to be any further or
closer than any other stars which is to
say that if you look at this star here
in the night sky and look at this star
here in the night sky when you look at
them they don't look that different they
look pretty much the same and if they
look the same
maybe one's brighter
maybe one's a little dimmer but they
don't look that far apart and so fine if
you imagine as the Greeks did what they
believed was that all these stars we're
basically the same distance away from
the earth they were all attached to a
celestial sphere a
imagine it as a large glass sphere that
surrounds the earth and all the Czar's
are attached to it and the celestial
sphere spun around the earth now this is
a model and this is how science works
models what does a model mean well a
model is basically a way to think about
the earth or think about the universe
overall it is a way to well I mean it
says model it is a model it is a
description of the universe around us
and it's a description which can provide
ok can provide a means to enhance our
understanding of the nature which is
what it says right there but it is a
means to understand nature that has
predictions predictions are important
predictions is what science is so
basically you come up with a model and
your model should tell us what is going
to happen in the future and if in the
future you predict something's gonna
happen
well that is science science predicts
what's gonna happen in the future and if
it turns out to be correct
if you're your model predicts what's
gonna happen then you have some
confidence in your model but
if your model predict something's gonna
happen in the future and it doesn't
happen well then your model is wrong
this is the basic foundation of science
and astronomy was the first of the
fields of science that was actually used
or and that's what's used but the first
field of science that was actually
developed and it's because in astronomy
or looking at the Stars you could
actually see the future now we will talk
more about this in detail but models and
science these are huge for this class
and we'll be talking it again in more
detail about it as we go on now
constellations I do want to talk about
constellations a little bit but not a
lot because constellations are basically
points on map constellations
historically have been used as sign
points in the sky so you pick a bunch of
stars usually bright stars and those
stars you name a name after something
that you know say a lion so in this case
is Leo the Lion or historical figures or
[Music]
imaginary figures things like Orion
constellations are basically a map you
can use constellations to figure out
where things are in the sky but you
should note that stars like this one
here
and this one here they look like they're
close together in the sky they are not
necessarily close together this guy they
can be very very far apart
okay constellations are only they only
look the way they do because of where we
are in space and we're gonna talk a lot
about that in the future okay like you
know our position in the galaxy because
we this in the galaxy all that stuff
that's coming up but from our point of
view here on the earth looking out into
space there are stars and from our point
of view of those stars and make a
pattern and we named those stars after
that pattern and this has been going on
for probably as long as humans have been
around okay so fine constellations they
are just stars in the sky we've named
them after certain mythological beasts
and real beasts and whatnot
constellations themselves are not that
important for modern astronomy they are
really just used as signposts hey this
is where you are in the sky
so now constellations over time well
over time
technically stars do move which is to
say that if you looked at say this star
right here if you look at this star
today you'll be right there tomorrow
right there but if you were to look at
the star say 100 years into the future
let's say a hundred years from now it
might have moved slightly so it might be
right there right here and if you look
at it again a thousand years from now it
would maybe move a bit further now
this kind of motion we call it proper
motion it has to do with the fact that
all stars orbit our galaxy our galaxy
the Milky Way galaxy there are hundreds
of billions of stars in the Milky Way
galaxy they all orbit around the center
of the galaxy kind of like you know
planets orbit the Sun stars orbit the
Milky Way and because of that and
because all stars have unique orbits
actually from our point of view all the
stars are moving but because they're so
far away because they're so far away
they only appear to move a little bit
and during your lifetime they will not
move much at all in fact they will move
almost from your point of view without a
telescope which is your eyeball they
won't move at all they won't move at all
period now the fact that they move over
time is something you're gonna be able
to see only over a long periods now if
you go back in time to like the ancient
Greeks and we'll be talking a lot about
the Greeks because they kind of codified
the idea of modern astronomy they look
at the Stars and they move and because
of that they thought the earth had to be
stationary now I want you to think about
the Stars and the fact that you know the
appearance over time and you know the
appearance over time I want you to think
about motion and motion with distance so
how far away a thing is and how much it
appears to move so think about this you
stand on the side of a rope
and a car goes by you well you can see
the car go by you it goes fast it
appears to move fast but what happens if
you look up into the sky and you see a
plane an airplane flying across the sky
way up in the sky look at it
that airplane appears to be moving very
slowly
is it moving slowly no it's actually
moving very fast it's actually moving
faster than the cars on the road much
faster the airplane is moving like five
hundred miles per hour the cars well
they should be moving much slower than
that right but the car moving past you
when it's only a few feet away looks
like it's moving west faster than the
airplane that's moving across the sky
very very far away from you and the deal
is the airplane is like these stars okay
it's very far away now the stars are
much much further away so even though
the stars are moving because they're so
far away during your lifetime you will
not notice that movement so the Swan
Cygnus this constellation is Cygnus the
Swan it will look like Cygnus the Swan
during your entire lifetime but say your
great-great-grandchildren
well the positions of these stars will
be slightly different not a lot
different again they are very far away I
mean the airplane the airplane in the
sky is maybe a few miles away mean at
most like five or six miles away
which is why it looks like it's moving
so slowly these stars are trillion miles
away trillion
of miles away hugely far away which is
why they don't seem to move all right
okay now when we look at the night sky
or day a time sky there are a couple of
things we notice first of all we notice
that there is daily motion so the Sun
rises the Sun sets the moon rises the
moon sets stars rise and they set and
now this has to do with the fact that
the earth is a ball it's a spherical
structure and it's spins on it and on an
axis that axis so if you take a ball and
you spin it it spins around
a an axis that points through the ball
now it the northern hemisphere in the
southern hemisphere are defined by this
motion so you have a sphere and you have
an axis and the sphere spins around that
axis and the northern hemisphere where
the axis comes out of the ball on the
north that is the North Pole where it
comes out in the South that is the South
Pole and the whole earth spins around
this axis about once every 24 hours so a
day now ancient astronomers assumed that
what was actually happening was that
things were spinning around the earth so
the earth was stationary in the center
and everything spun around the earth
okay and that is the idea of the
celestial sphere so all the stars all
the planets and the Sun were attached to
celestial spheres these the big things
that's
around the earth now today we know that
this daily motion is actually caused by
the fact the earth itself is spinning
once about every 24 hours
now this is not the only kind of motion
we see in the sky
it's just daily motion so you see the
sunrise and sunset that's because the
earth is spinning on its axis so
sometimes we're facing the Sun sometimes
we're not but it is also the case that
you have a Sun and you have the Earth
orbiting around the Sun so it goes
around the Sun once per year now what
this means is that different times of
the year so for instance if the earth is
here at night we will see stuff that is
in this direction because during the day
we're looking towards the Sun right at
night we're looking away from the Sun
now six months later the earth is over
here which means at night we're looking
in this direction right so we're seeing
different stuff at night different stuff
in the sky this is annual motion this
has to do with the fact that the Earth
orbits around the Sun once a year daily
motion has to do with the fact they were
spins on its axis that's why the Sun
rises and sets but the fact that we see
different stars in the sky at different
times of the year that has to do the
fact that we go around the Sun okay so
say we're here we're looking in this
direction if we are here we're looking
in this direction alright that makes
sense
hopefully so there are two different
kinds of motion that are happen
but the ancient Greeks and again bad
reasons for this we'll talk about the
reasons in particular they had for this
it had to do with stars and had to do
with the position of stars in the sky
they believed the earth was stationary
so the daily and annual motion that we
observe
they believed that happened because of a
celestial sphere that surrounded the
earth and we saw different stuff based
on how that sphere spun today we know
that's not true
but again they had good reasons and
we'll talk about now the motion again
the Earth spins on an axis now this axis
that the Earth spins on is tilted
relative to the Sun which is a say if we
are here in the northern hemisphere or
if here in the southern hemisphere
you're in this other hemisphere wherever
you are if you're not in the tropics
when you look at the Sun when it gets to
the highest point and where it gets in
the sky which technically we would call
that noon when the Sun gets the highest
point in the sky is it directly overhead
well if you're not in the tropics the
answer there should be no oh the Sun is
never directly overhead if you're not in
the tropics never okay fine the sun has
ever ahead why well because the earth is
tilted relative to its orbit around the
Sun so we orbit around the Sun imagine
that orbit as a frisbee a plane okay or
the the top of a desk we go around the
Sun on that desk on the top of that desk
okay now
the earth itself when it spins again you
can think of the earth as like a little
marble and you can spin that marble the
Earth's spin is tilted relative to the
Sun it's tilted twenty three and a half
degrees that is this tilt here and
because of that because of that tilt
when the Sun well first of all will
never be directly overhead unless you're
in the tropics near the earth's equator
but the earth's equator is tilted
relative to the the Earth's orbit around
the Sun and the Earth's orbit around the
Sun we call that the ecliptic and the
earth the earth itself when it spins
because it's tilted relative to that the
earth the center of the earth we call
that the equator and if you imagine the
earth's equator projected out into space
we call that the celestial equator but
it's tilted relative to what we call the
ecliptic now the ecliptic is where or
the Earth's orbit around the Sun that is
the ecliptic and from our point of view
here on the earth you can always tell
where they clip the gives just by
looking where is the Sun because the
Earth's orbit around the Sun is the
ecliptic well from our point of view the
Sun is always going to be on the
ecliptic so basically the ecliptic is
the sun's path through the sky and at
night you can imagine where the Sun
would be in the sky and that is the
ecliptic so fine
oscillations things that we see in the
sky what happens over time because we're
moving different constellations we
visible during the day or at night
rather and it's just you know the
direction we're facing great so if
here's the Sun we orbit around the Sun
at certain times the year we're looking
in this direction we see different
constellations we're looking in this
direction six months later we're gonna
see different constellations and when
they rise and set depends on how fast
the earth is actually spinning but what
constellations are in the sky depends on
where the earth is around the Sun all
right so constellations will change
yearly based on where we are so here
we're looking in this direction we're
gonna see certain constellations here at
night we're looking in this direction
will see different constellations now
what this means is you can use
constellations as a calendar and this is
huge this is what astronomy really was
historically astronomy used to be a
calendar it's how we kept track of time
all right so this is annual motion right
the earth going around the Sun at
different times of the year at night we
can see different constellations if you
could see constellations during the day
you can't but if you could what you
would see is the Sun appears to be in
particular constellations these
constellations are the zodiac now I do
not want you to that I do not want you
guys to imagine the zodiac somehow
controls your life it doesn't if you are
interested in a different kind of class
well you might be interested in
astrology but I will tell you this right
now astrology is wrong ridiculous and
silly this is non astrology this is
astronomy this is a science ok now why
are these constellations important yeah
there are these constellations you
probably have heard of them Aries
Capricorn Sagittarius Scorpius
okay these are the constellations of the
zodiac they are historically important
because they are a calendar okay if you
are on the earth in June during the day
the Sun is up and if you could see the
constellations as Sun is in well it's
between Taurus and Gemini but at night
Scorpius is up this constellation is up
it is the constellation that at midnight
is in the center of the sky and if you
know if you are familiar with the zodiac
and you see Scorpius in the center of
the sky well you know it's June it's a
calendar okay in August center of the
sky is gonna be in this direction and if
you sees hedges Terius and Capricorn and
you know where they should be in this
guy or you know that you're looking
right in the middle of those at midnight
well that's calendar it tells you it's
August that is what the zodiac
historically has been used for not to
predict what's gonna happen to you today
that's ridiculous it can't happen but it
does tell you what time of year it is
you can use it as a calendar so fine the
ecliptic
is the path of the Sun in our sky from
our point of view what's really going on
is the Earth orbits the Sun but from our
point of view the Sun is going to be in
a particular pop spot in the sky we call
that the ecliptic and it turns out that
if we could see constellations back here
well the Sun would be in some
constellation that is the zodiac and
that is the zodiac okay now that's it
for the first part of this lecture I
hope you guys are having a enjoyable
time we will continue to talk about how
things move in the sky and what that
means in the next couple of parts of
this first lecture so enjoy
5.0 / 5 (0 votes)